refactor: flag sanity checks, +feat: flag limits

- Added new config flag:limitPerTarget, to disallow flags after an item has
  already been flagged x times (default 0, or infinite)
- New zset flags:byTarget, score is the number of times a flag has been made
  against that item
- "already-flagged" translation key removed, now "post-already-flagged" or
  "user-already-flagged" -- this fixed bug where flagging a user you've already
  flagged would tell you you've already flagged this post already.
- Refactored Flags.canFlag to throw errors only, instead of returning boolean
- Updated ACP form inputs for reputation settings page to be more bootstrappy
- +1 upgrade script

install/data/defaults.json

@@ -79,6 +79,7 @@
     "min:rep:website": 0,
     "min:rep:aboutme": 0,
     "min:rep:signature": 0,
+    "flags:limitPerTarget": 0,
     "notificationType_upvote": "notification",
     "notificationType_new-topic": "notification",
     "notificationType_new-reply": "notification",